The present invention relates to document scanning and specifically to an apparatus that reduces show-through when a duplex-printed document (printed on both sides) is scanned but without producing undesirable artifacts such as black hole or black border.
Various scanning technologies are used in document scanners, copiers, and facsimile machines (collectively, “document-scanning systems”) to convert documents into a digital image for copying, document distribution, or archiving. (Throughout this application, the term “document” means any material that may be scanned, such as paper, vellum, plastic, cardboard, photographs, or similar materials.) Generally, a scanner lamp illuminates a document, and a scanner sensor detects the reflection at a single point (such as a pixel) in the document. Scanning technologies measure the relative brightness (ranging from black to white) of each pixel reflection. The detected brightness is converted into an analog signal that typically is subsequently converted into a digital signal. The term “digital count” refers to the measure of brightness of the reflection, where (for example) zero is least bright (black) and 255 is brightest (white). In this exemplary scheme, if the pixel is black, a minimal amount of light is reflected onto the scanner sensor (digital count=0). On the other hand, if the pixel is white, a maximum amount of light is reflected onto the scanner sensor for a digital count of 255.
Before scanning a document, a cover or backing (referred to generally throughout this application as a “document-backing surface”) is generally placed behind the document. It's well understood that the color of the document-backing surface is critical to the quality of scanned images. A document-backing surface may be white. With a duplex-printed document, however, the use of a white document-backing surface can create an inaccurate scanned image because the printing from the back side “shows through” and is visible in the scanned image. As an example, FIG. 1 shows a scanned image of a white document that has a kanji character printed on the back side. In the example shown, the area with back-side printing 20 has a reflectance that is about 5% (15 digital counts) lower than the area without back-side printing 22. As FIG. 1 illustrates, a difference of only 15 digital counts is large enough to perceptibly distort a scanned image. FIG. 1 also illustrates that show-through is undesirable. In some copiers, show-through is so severe that background-removal parameters must be adjusted to remove it. But parameter adjustment causes some highlight color loss in copying map originals. In addition, show-through can cause image-processing algorithms to fail. For example, a text region with show-through can easily be missegmented as a photo region due to reduced contrast.
To appreciate the show-through phenomenon, it is helpful to explain the path that light takes to reach a scanner sensor in a document-scanning system. FIG. 2 shows a schematic of an exemplary document-scanning system 24. In this document-scanning system 24, a document 28 on a platen glass 25 is illuminated by a scanner lamp 26 with incident light rays (I0) (shown as a single light ray). Reflected light is collected by a scanner lens 30 and focused on a scanner sensor 32. In the shown document-scanning system 24, document 28 is a sheet of white paper with back-side printing 36 and front-side printing 38. In FIG. 2, the scanner sensor 32 detects a reflection from an area of document 28 that has neither back-side nor front-side printing 36, 38. There are two main components of reflected light that reach the scanner sensor 32. The first component is first-reflected light rays (I1), which are created when incident light rays (I0) are scattered off document 28. The second component, as shown in FIG. 3, is second-transmitted light rays (I4). If document 28 is not opaque (a white-paper document is not opaque), part of the incident light rays (I0) pass through document 28 as first-transmitted light ray (I2) and are reflected off the document-backing surface 34, creating second-reflected light ray (I3). Part of second-reflected light ray (I3) is then retransmitted through document 28 as second-transmitted light rays (I4). The two components of reflected light, first-reflected light rays (I1) and second-transmitted light rays (I4), are collected by the scanner lens 30 and focused on the scanner sensor 32.
If an area of document 28 does not have back-side printing 36, second-transmitted light rays (I4) reach the scanner sensor 32 without creating show-through. If, however, the back side of document 28 contains back-side printing 36, as shown in FIG. 4, a portion of first-transmitted light ray (I′2) and second-reflected light ray (I′3) will be absorbed by the back-side printing 36. This absorption reduces the intensity of second-transmitted light rays (I′4). Show-through results when low-intensity second-transmitted light rays (I′4) are contrasted with the relatively bright second-transmitted light rays (I4) from areas without back-side printing 36.
The manner in which light reaches a scanner sensor 32 in a document-scanning system 24 may be expressed mathematically. The reflectance Rp detected by the scanner sensor 32 when scanning the front-side of paper document 28 is given by:Rp=(T2f(x,y)(Sp+Tp2RbkT2b(x,y))  Equation (1)where Tf is the transmittance of the front-side printing 38, x and y denote the coordinate position of the scanned pixel, Sp is the fraction of light scattered by document 28 in the forward direction, Tp is the transmittance of document 28, Rbk is the coefficient of reflection of document-backing surface 34, and Tb is the transmittance of back-side printing 36. In an area of a document 28 with back-side printing 36, the reflectance Rp depends on front-side printed pattern transmittance Tf and the back-side printed pattern transmittance Tb. This dependence on the printed pattern transmittance terms (Tf and Tb) is the cause of show-through in document scanning.
To assist the reader in understanding the present invention, it is helpful to provide an expression for the quantity of light (radiant flux Φ) collected by a scanner lens 30 in the exemplary document-scanning system 24 shown in FIG. 2. Irradiance (E) is given by the following expression:                               E          1                =                                            L              0                        ⁢                          r              0                        ⁢                          cos              ⁡                              (                θ                )                                      ⁢                                                   ⁢            π                                r            1                                              Equation        ⁢                                   ⁢                  (          2          )                    As shown in FIG. 2, r0 denotes the radius of the scanner lamp 26 with radiance of L0 (watts per steradian per meter-squared). The distance from the scanner lamp 26 to the pixel scanned is r1. The scanner lamp 26 illuminates the scanned pixel in document 28 at an angle of θ degrees from the optical axis 84 of the scanner sensor 32. Light with irradiance E1 is reflected by document 28 with a coefficient of reflectance Rp. If it is assumed that document 28 is a Lambertian diffuse reflector, light reflected by document 28 has a radiance L1 given by:                               L          1                =                                                            E                1                            ⁢                              R                p                                      π                    .                                    Equation        ⁢                                   ⁢                  (          3          )                    The radiant flux collected by scanner lens 30 from the reflection off document 28 is:                                                                         Φ                1                            =                            ⁢                                                                    L                    1                                    ⁢                  d                  ⁢                                                                           ⁢                  A                  ⁢                                                                           ⁢                  d                  ⁢                                                                           ⁢                  Ω                                =                                                                                                    L                        0                                            ⁢                                              R                        p                                            ⁢                                              cos                        ⁡                                                  (                          θ                          )                                                                    ⁢                                              r                        0                                                                                    r                      1                                                        ⁢                                                                           ⁢                  d                  ⁢                                                                           ⁢                  A                  ⁢                                                                           ⁢                                                            π                      ⁢                                                                                           ⁢                                              D                        2                                                                                    4                      ⁢                                              r                        2                        2                                                                                                                                                                    =                            ⁢                                                                    L                    0                                    ⁢                                      R                    p                                    ⁢                                      cos                    ⁡                                          (                      θ                      )                                                        ⁢                                      r                    0                                    ⁢                  π                  ⁢                                                                           ⁢                                      D                    2                                    ⁢                  d                  ⁢                                                                           ⁢                  A                                                  4                  ⁢                                      r                    1                                    ⁢                                      r                    2                    2                                                                                                          Equation        ⁢                                   ⁢                  (          4          )                    In Equation (4), the distance from the scanned pixel to scanner lens 30 is r2 (shown in FIG. 2) and D is the diameter of the scanner lens 30 (not shown). The area of the scanner sensor element 32, projected onto the paper document 28 is dA (also called the instant-field-of-view or “IFOV”).
Equation (4) is an expression for the radiant flux reflected from a document 28. In addition, it is helpful to have an expression for the radiant flux reflected from a specularly reflective surface 40. FIG. 5 shows a schematic of an exemplary configuration 41 in which a lamp 27 (of radiance L0) illuminates a specularly reflective surface 40. The radiant flux focused on a sensor 33 by a lens 31 (of diameter D) from the reflection off specularly reflective surface 40 is:                               Φ          2                =                                            L              0                        ⁢            d            ⁢                                                   ⁢            A            ⁢                                                   ⁢            d            ⁢                                                   ⁢            Ω                    =                                                    L                0                            ⁢                              R                m                            ⁢              d              ⁢                                                           ⁢              A              ⁢                                                           ⁢                                                π                  ⁢                                                                           ⁢                                      D                    2                                                                    4                  ⁢                                      r                    2                    2                                                                        =                                          π                ⁢                                                                   ⁢                                  L                  0                                ⁢                                  R                  m                                ⁢                                  D                  2                                ⁢                d                ⁢                                                                   ⁢                A                                            4                ⁢                                  r                  2                  2                                                                                        Equation        ⁢                                   ⁢                  (          5          )                    where Rm is the coefficient of reflectivity of the specularly reflective surface 40 and is very close to 1 (100%), and dA is the IFOV of sensor 33.
Turning now to the known techniques for improving the quality of scanned images, several prior art references are reviewed.
U.S. Pat. No. 6,101,283 to Knox, U.S. Pat. No. 5,832,137 to Knox, and U.S. Pat. No. 5,646,744 to Knox (“the Knox references”) disclose an image-processing method for show-through suppression. The scanned back-side image of a document is used to create a representation of the show-through contribution to the front-side image. The representation is then used to suppress the show-through in the front-side image. One drawback of the Knox references is that both sides of a document must be scanned. This requires an additional scanner (if both sides are scanned simultaneously) or additional processing steps (to flip the document and scan its back-side). A problem with a duplex scanning approach is that it is too costly to be practical. Moreover, the image-processing algorithms are complex. In addition, front- and back-side scanned images must be aligned. If dual scanners are not employed, human guidance may be required to align the scanned images.
Show-through can be substantially reduced if the document-backing surface is black. Referring to the above expression (Equation (1)) for the reflectance (Rp) of a paper document 28, if the document-backing surface is black (i.e., Rbk→0), the reflection is reduced to near zero, and only the reflection (Sp) off the front side of the document 28 is seen by the scanner sensor 32.
U.S. Pat. No. 5,053,818 to Smith (“the Smith reference”) takes advantage of this property by placing a black sheet of backing paper behind the document being scanned. Specifically, the Smith reference discloses a method for (1) detecting thin original documents (i.e., documents with potential show-through risk), (2) automatically printing a black page (or feeding a black page from a tray of black paper) and placing it between the thin original document and the document-backing surface, and (3) adjusting the copy lightness setting to eliminate the uniformly gray background. The Smith reference, however, suffers from all the problems (described below) that are inherent in the use of a black background.
FIG. 6 shows an example of a scanned image 86 that results from scanning a document 28 against a black background. As FIG. 6 shows, if the document 28 is smaller than the defined area to be scanned, an undesired black border 42 is produced in the scanned image. Similarly, if holes 43a, tears 43b, dog-ears 43c, or other defects are part of a document 28, these defects show up as undesired black areas in the scanned image. Black border 42, holes 43a, tears 43b, dog-ears 43c, or other defects are collectively referred to herein as “artifacts” 44.
Prior art solutions directed to removal of artifacts include U.S. Pat. No. 6,078,051 to Banton (“the Banton reference”) and U.S. Pat. No. 6,122,393 to Schweid et al. (“the Schweid reference”). The Banton and Schweid references are related references that disclose a document-backing surface with a pattern that generates a distinct reflection when scanned. In addition, a means for estimating whether a particular reflection is from the document being scanned or the document-backing surface is disclosed. The requirement for an estimating means increases the cost and complexity of the disclosed apparatus. Further, the algorithm used in the estimating means is susceptible to failure which can cause false corrections, e.g., human eyes may be detected as holes and removed from the image.
Another prior art solution directed to artifact removal is U.S. Pat. No. 5,987,270 to Hulan et al. (“the Hulan reference”), which discloses a copier that scans a defined field of view and searches the scanned image for reference markings and operational components visible on the underside of an automatic document-feeder cover. If a reference marking, operational feature, or shadow is found, it is suppressed in the document image. Further, reference markings are used to determine the size of the document to be copied, and all images scanned outside the document size are suppressed. One disadvantage of the Hulan reference is that it does not address the problem of show-through. Another drawback is that holes, tears, and dog-ears may show up as black artifacts. An additional limitation is that a means for identifying reference markings and operational components is required. This increases the cost and complexity of the disclosed apparatus.
U.S. Pat. No. 6,166,394 to Rubscha (“the Rubscha reference”) is yet another example of a prior art solution directed to artifact removal. The Rubscha reference discloses a scanning system in which a document is moved past a sensor. The imaging of undesired document apertures or openings is eliminated by mechanically moving either the image-sensing unit or a backing baffle so as to provide alternately a white or black background. At start-up, a black background is provided for sheet-edge detection. For scanning, a white background is provided. Because a white background is used for scanning, duplex documents scanned using the Rubscha reference may suffer from show-through. A further disadvantage of the Rubscha reference is that it relies on moving parts, which are susceptible to mechanical breakdown.